It is known to heat synthetic plastics materials by infra-red radiation to enable moulding thereof to be effected. Preferably, infra-red radiators which permit very short heating periods, lasting only a few seconds but of great intensity are used for this purpose. Infra-red radiation offers the advantage that it is not only the surface of the irradiated plastics material that is heated but that the deeper layers are also directly reached through the surface. Nevertheless, the radiation is predominantly absorbed by that portion of the material that is nearest to the source of radiation and thus, with intensive irradiation, a high-temperature peak is thereby caused on the irradiated surface. Due to the low thermal conductivity of plastics materials, a temperature balance within the material occurs, in the case of infra-red radiation, only relatively slowly during the short heating period.
This lack of temperature balance frequently leads to undesired changes in the overheated surface layer, particularly when workpieces with a large wall thickness have to be heated for a somewhat longer time than usual, in order to introduce the necessary quantity of thermal energy into the workpiece.
An object of the invention is to prevent changes in the surface layer, caused by an unnecessarily high degree of heating, in thick-walled workpieces of plastics material which are to be heated to enable subsequent moulding thereof to be effected.